The present invention is a method for densifying particulate silica and a method for compounding a curable silicone composition comprising the densified particulate silica. The method is particularly useful for densifying particulate silica to be used as a continuous feed to a compounding apparatus for forming curable silicone compositions.
Particulate silica is a well know reinforcing filler commonly employed to improve the physical properties of organic rubber and silicone rubber and sealants. The particulate silica can typically have a surface area within a range of about 50 m.sup.2 /g to 1000 m.sup.2 /g and a pour bulk density of about 25 kg/m.sup.3 to 64 kg/m.sup.3. A major disadvantage of such low bulk density silica is that it is relatively expensive to ship and store. An additional disadvantage of such low bulk density silica is that an inability to feed sufficient amounts to continuous compounding equipment to achieve desired silica levels may limit the capacity of the equipment. According it is desirable to provide a means for increasing the bulk density of particulate silica in order to overcome the above described shortcomings, while at the same time not altering the silica so as to compromise the properties imparted to compositions in which it is incorporated.
There are known mechanical means for increasing the bulk density of a particulate material. Oldham et al., U.S. Pat. No. 3,114,930, teach the use of a vacuum to remove air from aerated powdered materials thereby increasing the density.
Carter, U.S. Pat. No. 3,664,385, teaches compacting finely divided particles by utilizing a rotating screw feeder positioned in a vertically orientated housing having a porous sleeve to which a vacuum is applied.
Kongsgarreden, U.S. Pat. No. 4,126,424, teaches a method for compacting silica dust where the dust is charged to a cylindrical hopper and then is injected with air under pressure to densify the dust.
Kongsgaarden, U.S. Pat. No. 4,126,423, teaches a method for increasing the bulk density of silica dust using a drum having internal ribs.
Loffler, U.S. Pat. No. 3,632,247, teaches a process where powders are compressed and deaerated between vacuum cylinders which are arranged in groups requiring different vacuum and connected to a common vacuum line.
Leon et al., U.S. Pat. No. 4,325,686, teach a powder densifying apparatus comprising a pair of opposed gas-permeable belts arranged to either side of a common axis so as to define a generally convergent densifying zone between their adjacent faces.
Kratel et al., U.S. Pat. No. 4,326,852, teach a method for increasing the bulk weight of silicon dioxide by means of vacuum pressure applied at a filter face where the silicon dioxide is moved by means of a conveyor screw whose longitudinal axis is arranged parallel with respect to the filter face and which preferably has a decreasing thread pitch in the feeding direction. Kratel et al. teach the use of the densified silicon dioxide as a filler for polymeric masses including polydiorganosiloxanes.
Klingle et al., U.S. Pat. No. 4,877,595, teach the compression of pyrogenically prepared silica by a rotary filter equipped with a pressing band.
Graville et al., U.S. Pat. No. 5,160,470, teach a method for densifying colloidal silica recovered from a smelting process. The method comprises feeding the silica by a horizontal feed means to a pair of vertically juxtaposed pressure rolls having surface depressions.
Razzano, U.S. Pat. No. 4,780,108, teaches a method for increasing the bulk density of low bulk density materials, such as silica, by intimately admixing a low bulk density material and an organic or polyorganosiloxane liquid and thereafter removing substantially all of the organic or polyorganosiloxane liquid by, for example, evaporation. Razzano teaches that a problem typically associated with mechanical methods for densifying silica is the formation of unacceptably high amounts of agglomerates and grit. Razzano further describes how such silicas no longer disperse as well in silicone compositions as silicas with lower bulk density.
Johanson, U.S. Pat. No. 5,052,874, describes an apparatus for densifying particulate solids useful in the present invention. Although Johanson describes the use of the apparatus for densifying particulate solids, he has not identified the specific use for densifying particulate silica nor that such use provides for densified silica having improved reinforcing capacity in rubber compositions when compared to silica densified by certain other mechanical means of densification.